A driver's perception of a vehicle's "driveability" is greatly influenced by the response felt when depressing the accelerator pedal to open the throttle valve.
During the initial opening of the throttle valve, so called "off-idle" maneuvers such as accelerating from a stop (especially on a slippery surface) or backing up, the ideal throttle valve control system should open slowly, i.e., a minimum number of degrees for a given length of linear accelerator cable travel. Opening the throttle valve slowly during off-idle maneuvers prevents the vehicle from breaking traction or lurching forward and allows the driver to easily modulate takeoff acceleration during off-idle and low speed maneuvers.
During mid-range throttle operation, such as accelerating to merge with traffic or to pass another vehicle, the ideal throttle valve control system should open the throttle valve at a faster rate, i.e., a greater number of degrees for the same amount of linear accelerator cable travel. Opening the throttle valve at a faster rate during mid-range operation provides the driver with the perception that the vehicle is responsive and that vehicle acceleration is readily available without having to fully depress the accelerator pedal.
In vehicles with an automatic transmission, the vehicle accelerator linkage actuates a downshift circuit during full-throttle range operation. To notify the driver that downshifting is about to occur, the ideal throttle valve control system should provide the driver with a slight but discernable increase in the force required to depress the accelerator pedal before downshifting occurs. To maintain the driver's perception that the accelerator system is predictable and controllable throughout the remainder of the full-throttle range, the driver should feel an approximately linear relationship in the force required to depress the accelerator pedal and the opening of the throttle valve.
In addition, the ideal control system should provide sufficient mechanical advantage between the point that downshifting occurs and the point that wide-open throttle is obtained to allow the driver to modulate the accelerator pedal travel and feel the difference between the effort required to initiate downshifting and the effort required to obtain wide-open throttle.